I'd do two similar complimentary patterns, one in the dark pieces and then one in the background block that connects them. I don't know how good you are at FMQ, but this one just screams Green Fairy Quilts type quilting to me. Basically ruler work that mirrors some of the seams and then filled in with a filler.
Or you could echo the black block design on the background blocks...
Or you could use the black block plus the next piece out to create a bigger X and then use the rest as a background -- that would leave a cool place (the diamond 4 patch) to do a different pattern.
I'd be tempted to do Orange peel on your border blocks and then something intricate matching the middle on the solid sashing.
But I tend to go pretty custom and can't really do edge to edge well enough on a sitdown.